I believe the original question mentioned "affordable" - the 400mm f5.6 is an incredible value. Though without IS, it is a very lightweight and maneuverable. Very popular for birds-in-flight, and just razor sharp for any telephoto need. Remember, you will almost always be using a 400mm lens on a tripod - so you can work around the IS and f5.6 issues while you save for something more expensive.
Of note is the fact this lens focuses very quickly. Also of note, I find when I set out with a super-telephoto (I once owned the Sigma 50-500), I am almost exclusively at the long end. This convinced me a prime 400 or 500 mm is totally justified and often used. It also steered me away from the 100-400 option (more expensive than a 400mm f5.6). When going less than super-telephoto, you can get by with a 70-200.